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7.1
NATURE OF LIGHT:
ELECTROMAGNETIC WAVES James Clerk Maxwell and Heinrich Rudolf Hertz
The theory of electromagnetism developed by the
great Scottish physicist James Clerk Maxwell between 1865 and 1873. He was
interested in the effects of oscillating electric currents in conductors. He
proposed that alternating currents in wires would set up fluctuating electric
and magnetic fields in the region surrounding the wire to produce waves with
a frequency equal to the frequency of the current oscillations. Maxwells
theory predicted that the radiated waves would behave in just like light.
These electromagnetic waves would exhibit:
reflection by metal mirrors; refraction by insulators (dielectrics)
such as glass; polarization effects; interference effects; and travel away
from the wire through a vacuum with a speed of 3.0x108 m.s-1.
Hence, Maxwell was led to the to the simplifying and unifying hypothesis that
light was a type of Maxwell wave or electromagnetic disturbance, created by
extremely high frequency electric oscillators in matter. But at this time, the
technology was lacking to provide experimental evidence to support Maxwells
hypothesis. In 1887 (about 15 years after Maxwell postulates were published),
in a series of brilliant and exhaustive experiments, Heinrich Hertz showed
that Maxwells theory was correct and that an oscillating electric current
does indeed radiate electromagnetic waves that possess every characteristic
of light except the frequency of Maxwells waves were many order of magnitude
lower than the frequency of a light wave. Hertz used a simple spark gap oscillator consisting
of two short rods attached to small metal spheres that were separated by an
air gap of a few millimeters. He applied pulses of high
voltage, which caused a spark to jump across the gap and produce a high
frequency electric
oscillation with frequency about 5x108 Hz.
He used a
simple loop antenna with a small spark gap as the receiver. Hertz very
quickly succeeded in detecting the radiation from his spark gap oscillator,
even at distances of several hundred meters and measured the wavelength from
a standing wave pattern to be about 600 mm, hence a frequency of 5x108
Hz In an
exhaustive series of experiments, Hertz show that the transmitted waves from
his electrical oscillator could be reflected, refracted, focused, polarized, and
made to interfere. He provided conclusive evidence that his Hertzian waves
and light waves were one and the same. This classical model of light as an
electromagnetic wave was quickly adopted by physicists at this time. It was
an impressive victory for Maxwell, it was found that Maxwells equations
correctly predicted the behavior of light and other electromagnetic waves.
Thus, through Maxwell theories and the experimental evidence from Hertz, the
formerly independent kingdoms of electricity, magnetism, and light! were
unified.
So, we can
conclude that through the efforts of Maxwell and Hertz, that light is a wave.
But, Hertz accidently discovered the photoelectric effect. To explain the
behavior of light and the observation of light impacting on a metal surface,
we must regard the light not as a wave, but a particle called a photon. Hertz in
describing his spark gap transmitter, he emphasizes that it is essential that the pole surfaces of
the spark gap should be frequently repolished
to ensure reliable operation of the spark. At first, this result was a
mystery to Hertz. To resolve the mystery, he later concluded that it was the
ultraviolet light from the initial spark acting on a clean metal surface that
caused current to flow more freely between the poles of the spark gap. The
ultraviolet light liberated electrons to enhance the current effects in the
spark gap. In the
process of verifying the electromagnetic wave theory of light, Hertz had discovered
the photoelectric effect, a phenomenon that would undermine the priority of
the wave theory of light and establish the particle theory of light on an equal
footing.
Ian Cooper Honorary Lecturer, School of Physics, University of Sydney ian.cooper@sydney.edu.au If you have any comments, suggestions or corrections please email Ian
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